1. Field of the Invention
The present invention relates to a metal powder, a conductive paste using the metal powder, a monolithic ceramic electronic component in which internal electrodes are formed using the conductive paste, and to a method for producing the metal powder.
2. Description of the Related Art
A monolithic ceramic electronic component, such as a monolithic ceramic capacitor, usually includes a ceramic laminate comprising dielectric ceramic layers, internal electrodes provided between the dielectric ceramic layers, and terminal electrodes electrically connected to the internal electrodes.
The ceramic laminate is formed by firing a green ceramic laminate in which a plurality of green ceramic sheets composed of a dielectric ceramic material are laminated. In order to form the internal electrodes, a conductive paste is printed on a plurality of the green ceramic sheets, and then the conductive paste is fired simultaneously with the green ceramic laminate. One of the ends of each internal electrode is exposed to one end face of the ceramic laminate. A conductive paste is applied to both end faces of the ceramic laminate so as to be connected to the exposed ends of the internal electrodes, followed by baking, to produce the terminal electrodes.
As described above, a conductive paste is used for forming internal electrodes of monolithic ceramic electronic components, in particular, monolithic ceramic capacitors. The conductive paste contains a metal powder which acts as a conductive constituent, and recently, noble metal powders such as silver powder or palladium powder, and base metal powders such as nickel powder or copper powder, have been used. As the size and thickness of monolithic ceramic electronic components decrease, a demand for production of fine metal powder, which is contained in conductive paste used for forming internal electrodes of the monolithic ceramic electronic components, is arising.
Conventionally, methods, such as a vapor phase process, for producing fine metal powder having a small particle size are used. However, since the cost of producing metal powder by the vapor phase process is high, there is a demand for producing metal powder having a small particle size by a liquid phase process.
For example, Japanese Examined Patent Application Publication No. 6-99143 discloses a method for producing metal powder by a liquid phase process in which a nickel salt solution is reduced in the liquid phase using a borohydride solution, such as a sodium borohydride solution, as the reducing agent, in order to obtain a metal powder having a small particle size.
However, since the boron in the reducing agent solution in the method disclosed in Japanese Examined Patent Application Publication No. 6-99143 is precipitated as an alloy or impurity in the metal powder, the purity of the resulting metal powder may be reduced. Consequently, the electrical properties of the metal powder are degraded, thereby making it unsuitable for use as the conductive constituent for forming internal electrodes in monolithic ceramic electronic components.
In order to inhibit the inclusion of impurities during the production of metal powder by a liquid phase process, for example, Japanese Unexamined Patent Application Publication No. 5-43921 discloses a method in which a solution containing basic nickel carbonate is reduced using hydrazine as the reducing agent. Since hydrazine is used as the reducing agent, impurities are not substantially precipitated in the metal powder during the reduction reaction. However, the minimum average particle size of the metal powder obtained by this method is 100 nm, which is too large for use as the conducting constituent to be contained in conductive paste for forming internal electrodes used for thinner monolithic ceramic electronic components.